WASHINGTON — A key former F.B.I. official cast doubt on the Justice Department’s case for dropping a criminal charge against President Trump’s former national security adviser Michael T. Flynn during an interview with investigators last week, according to people familiar with the investigation. Department officials reviewing the Flynn case interviewed Bill Priestap, the former head of F.B.I. counterintelligence, two days before making their extraordinary request to drop the case to Judge Emmet G. Sullivan. They did not tell Judge Sullivan about Mr. Priestap’s interview. A Justice Department official said that they were in the process of writing up a report...